Interesting papers: Part I

April 21, 2011

[1] Influence of dislocation density on the pop-in behavior and indentation size effect in CaF2 single crystals: Experiments and molecular dynamics simulations

M A Lodes et al

In this work, the indentation size effect and pop-in behavior are studied for indentations in undeformed and locally pre-deformed CaF2 single crystals, using both nanoindentation experiments and molecular dynamics simulations. To study the influence of dislocation density on the indentation behavior, small-scale indentations are carried out inside the plastic zone of larger indentations. This experiment is mimicked in the simulations by indenting a small sphere into the center of the residual impression of a larger sphere. The undeformed material shows the well-known pop-in behavior followed by the indentation size effect. Pre-deforming the material leads to a reduction in the indentation size effect both for experiments and simulations, which is in accordance with the Nix–Gao theory. Furthermore, the pop-in load is reduced in the experiments, whereas a smooth transition from elastic to plastic deformation is found in the simulations. There, plasticity is initiated by the movement of pre-existing dislocation loops in the vicinity of the plastic zone. The simulations thus give a detailed insight into the deformation mechanism during indentation and highlight the importance of the dislocation microstructure for the indentation size effect and dislocation nucleation.

[2] Evolution equations and size distributions in nanocrystalline grain growth

Streitenberger and Zoellner

Size effects observed in nanocrystalline grain growth are modelled by attributing a specific energy and finite mobility to each structural feature of a polyhedral grain. By considering grain growth as a dissipative process that is driven by the reduction in the Gibbs free interface, edge and vertex energy, a general grain evolution equation is derived that can be divided into nine types of possible growth kinetics. The corresponding self-similar grain size distributions are derived and compared with results from modified Monte Carlo Potts model simulations taking into account size effects in triple and quadruple junction limited grain growth.

[3] Surface eigen-displacement and surface Poisson’s ratios of solids

Zhang et al

Theoretical analysis and molecular dynamics simulations were conducted to study systematically surface eigen-displacement and surface Poisson’s ratios of solids, which play essential roles in surface energy, surface strain and surface stress. Face-centered cubic (0 0 1) Au thin films were taken as typical examples to illustrate the physical picture. The surface eigen-displacement is a critical surface strain at the equilibrium state after normal relaxation and thus an intrinsic surface property. Surface Poisson’s ratios are also intrinsic surface properties. Combining surface eigen-displacement and surface Poisson’s ratios with surface eigen-stress and surface tangential elastic constants lays foundations of surface elasticity of solids.
Highlights

► Surface eigen-displacement is a critical surface strain at the equilibrium state after normal relaxation. ► Surface Poisson’s ratio is the surface excess of Poisson’s ratio. ► Surface eigen-displacement and Surface Poisson’s ratio are surface intrinsic properties.

[4] Thermodynamic assessment of the stabilization effect in deformed shape memory alloy martensite

Kato et al

When a martensitic shape memory alloy is deformed, the reverse transformation occurs at higher temperature than that of undeformed martensite. This is a typical case of the stabilization effect of martensite that is commonly observed in shape memory alloys. Regarding previous results measured by electric resistance and/or dilatometoric methods in NiTi and CuAlNi shape memory alloys, this study has performed calorimetric measurement in these alloys in order to re-examine the stabilization effect in terms of thermodynamics. Experimental evidence for appreciable changes in the reverse transformation temperature due to variant change of the martensite is presented. The elastic energy stored in the deformed martensite and the irreversible energy dissipated during the reverse transformation are estimated from the transformation temperatures, the stress–strain curves of the martensite and the latent heat of transformation. The temperatures of the reverse martensitic transformation have been related to these energies in explicit form.

[5] Quantitative analysis of layering and in-plane structural ordering at an alumina–aluminum solid–liquid interface

Kauffmann et al

Real-time observations of Al–Al2O3 dynamic liquid–solid interfaces on the atomic scale indicate the presence of structural ordering in the liquid at the solid–liquid interface. The main problem with direct high resolution transmission electron microscopy (HRTEM) interpretation is that the imaging conditions and aberrations in the imaging system have a significant influence on the contrast in the image, and may lead to inaccurate conclusions about the structure examined. New quantitative results based on using a single image iterative wave function reconstruction are presented. This technique requires only a single experimental image, and allows extraction of reliable and aberration-free structural information from experimental HRTEM micrographs. This numerical phase retrieval method was successful in analysis of the experimental data and allowed, for the first time, direct extraction of quantitative information regarding the degree of ordering (parallel and perpendicular to the interface) at liquid–solid interfaces. The degree of ordering at the Al2O3–Al interface at 750 °C was quantified and both layering and in-plane ordering were found. The layering in the liquid extends to about four to five layers (about 1 nm from the edge of the crystal). The in-plane ordering, which was observed only in the first three layers of the liquid, decays faster than the layering. In addition, the interlayer spacings measured in the liquid indicate that the liquid atoms at the interface are influenced by the structure of the crystal, while further away the ordering of the liquid atoms gradually disappears, until they adopt the characteristics of the bulk liquid.
Highlights

► In-situ HRTEM of Al–Al2O3 liquid–solid interfaces show ordering in the liquid at the interface. ► Quantitative results using a single image iterative wave function reconstruction are presented. ► Both layering and in-plane ordering exist at the interface at 750 °C. ► Layering extends to 4–5 liquid layers. In-plane ordering exists in the first 3 layers of the liquid.

[6] Phase field theory of proper displacive phase transformations: Structural anisotropy and directional flexibility, a vector model, and the transformation kinetics

Rao and Khachaturyan

A phase field theory of proper displacive transformations is developed to address the microstructure evolution and its response to applied fields in decomposing and martensitic systems. The theory is based on the explicit equation for the non-equilibrium free energy function of the transformation strain obtained by a consistent separation of the total strain into transformation and elastic strains. The transformation strain is considered to be a relaxing long-range order parameter evolving in accordance with the system energetics rather than as a fixed material constant used in the conventional Eshelby theory of coherent inclusions. The elastic strain is defined as a coherency strain recovering the crystal lattice compatibility. The obtained free energy function of the transformation strain leads to the concepts of structural anisotropy and directional flexibility of low symmetry phases. The formulated vector model of displacive transformation makes apparent a similarity between proper displacive transformation and ferromagnetic/ferroelectric transformation and, in particular, a similarity between the structural anisotropy and magnetic/polar anisotropy of ferromagnetic/ferroelectric materials. It even predicts the feasibility of a glass-like structural state with unlimited directional flexibility of the transformation strain that is conceptually similar to a ferromagnetic glass. The thermodynamics of the equilibrium be

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